/*
 * Copyright 2002-2017 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.scheduling.annotation;

import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.Future;
import java.util.concurrent.TimeUnit;

import org.springframework.lang.Nullable;
import org.springframework.util.concurrent.FailureCallback;
import org.springframework.util.concurrent.ListenableFuture;
import org.springframework.util.concurrent.ListenableFutureCallback;
import org.springframework.util.concurrent.SuccessCallback;

/**
 * A pass-through {@code Future} handle that can be used for method signatures
 * which are declared with a {@code Future} return type for asynchronous execution.
 *
 * <p>As of Spring 4.1, this class implements {@link ListenableFuture}, not just
 * plain {@link java.util.concurrent.Future}, along with the corresponding support
 * in {@code @Async} processing.
 *
 * <p>As of Spring 4.2, this class also supports passing execution exceptions back
 * to the caller.
 *
 * @author Juergen Hoeller
 * @author Rossen Stoyanchev
 * @see Async
 * @see #forValue(Object)
 * @see #forExecutionException(Throwable)
 * @since 3.0
 */
public class AsyncResult<V> implements ListenableFuture<V> {

    @Nullable
    private final V value;

    @Nullable
    private final Throwable executionException;


    /**
     * Create a new AsyncResult holder.
     *
     * @param value the value to pass through
     */
    public AsyncResult(@Nullable V value) {
        this(value, null);
    }

    /**
     * Create a new AsyncResult holder.
     *
     * @param value the value to pass through
     */
    private AsyncResult(@Nullable V value, @Nullable Throwable ex) {
        this.value = value;
        this.executionException = ex;
    }


    @Override
    public boolean cancel(boolean mayInterruptIfRunning) {
        return false;
    }

    @Override
    public boolean isCancelled() {
        return false;
    }

    @Override
    public boolean isDone() {
        return true;
    }

    @Override
    @Nullable
    public V get() throws ExecutionException {
        if (this.executionException != null) {
            throw (this.executionException instanceof ExecutionException ?
                    (ExecutionException) this.executionException :
                    new ExecutionException(this.executionException));
        }
        return this.value;
    }

    @Override
    @Nullable
    public V get(long timeout, TimeUnit unit) throws ExecutionException {
        return get();
    }

    @Override
    public void addCallback(ListenableFutureCallback<? super V> callback) {
        addCallback(callback, callback);
    }

    @Override
    public void addCallback(SuccessCallback<? super V> successCallback, FailureCallback failureCallback) {
        try {
            if (this.executionException != null) {
                failureCallback.onFailure(exposedException(this.executionException));
            } else {
                successCallback.onSuccess(this.value);
            }
        } catch (Throwable ex) {
            // Ignore
        }
    }

    @Override
    public CompletableFuture<V> completable() {
        if (this.executionException != null) {
            CompletableFuture<V> completable = new CompletableFuture<>();
            completable.completeExceptionally(exposedException(this.executionException));
            return completable;
        } else {
            return CompletableFuture.completedFuture(this.value);
        }
    }


    /**
     * Create a new async result which exposes the given value from {@link Future#get()}.
     *
     * @param value the value to expose
     * @see Future#get()
     * @since 4.2
     */
    public static <V> ListenableFuture<V> forValue(V value) {
        return new AsyncResult<>(value, null);
    }

    /**
     * Create a new async result which exposes the given exception as an
     * {@link ExecutionException} from {@link Future#get()}.
     *
     * @param ex the exception to expose (either an pre-built {@link ExecutionException}
     *           or a cause to be wrapped in an {@link ExecutionException})
     * @see ExecutionException
     * @since 4.2
     */
    public static <V> ListenableFuture<V> forExecutionException(Throwable ex) {
        return new AsyncResult<>(null, ex);
    }

    /**
     * Determine the exposed exception: either the cause of a given
     * {@link ExecutionException}, or the original exception as-is.
     *
     * @param original the original as given to {@link #forExecutionException}
     * @return the exposed exception
     */
    private static Throwable exposedException(Throwable original) {
        if (original instanceof ExecutionException) {
            Throwable cause = original.getCause();
            if (cause != null) {
                return cause;
            }
        }
        return original;
    }

}
